Understanding the processes that lead from a fertilized egg to the formation of germ layers and subsequently to a body plan is a central goal of embryology. Much of what is known about the development of a vertebrate body plan comes from studies of amphibia where, at the tadpole stage, the main body axis consists of the dorsal structures notochord, spinal cord and somites organized anterior to posterior as head, trunk and tail. All animal tissues derive from the three germ layers and the mesoderm plays a pivotal role in organizing the body axis (Keller, R. in Methods in Cell Biology, eds Kay and Peng, Academic Press: San Diego, 1991). Mesodermal cells lead the movements of gastrulation (Keller et al. (1988) Development 103:193-210; and Wilson et al. (1989) Development 105:155-166), are required for the patterning of the nervous system (Mangold et al. (1933) Natyrwissenschaften 21:761-766; and Hemmati-Brivanlou et al. (1990) Science 250:800-802), and themselves give rise to the muscular, skeletal, circulatory and excretory systems. Moreover, a portion of the dorsal mesoderm from early gastrula, the Spemann organizer, can induce and organize a second body axis following transplantation to another site (Spemann et al. (1924) Arch mikr Anat EntwMech 100:599-638).
The origin of the nervous system in all vertebrates can be traced to the end of gastrulation. At this time, the ectoderm in the dorsal side of the embryo changes its fate from epidermal to neural. The newly formed neuroectoderm thickens to form a flattened structure called the neural plate which is characterized, in some vertebrates, by a central groove (neural groove) and thickened lateral edges (neural folds). At its early stages of differentiation, the neural plate already exhibits signs of regional differentiation along its anterior posterior (A-P) and mediolateral axis (M-L). The neural folds eventually fuse at the dorsal midline to form the neural tube which will differentiate into brain at its anterior end and spinal cord at its posterior end. Closure of the neural tube creates dorsal/ventral differences by virtue of previous mediolateral differentiation. Thus, at the end of neurulation, the neural tube has a clear anterior-posterior (A-P), dorsal ventral (D-V) and mediolateral (M-L) polarities (see, for example, Principles in Neural Science (3rd), eds. Kandel, Schwartz and Jessell, Elsevier Science Publishing Company: NY, 1991; and Developmental Biology (3rd), ed. S. F. Gilbert, Sinauer Associates: Sunderland Mass., 1991).
Before gastrulation the three germ layers are simply arranged, top to bottom, in a frog blastula. Ectoderm arises from the top, or animal pole; mesoderm from the middle, or marginal zone, and endoderm from the bottom or vegetal pole. Mesoderm can be induced in animal pole cells (animal caps) by signals emanating from the vegetal pole. Several peptide growth factors have been identified that can induce mesoderm in animal caps in vitro. When animal cap tissue is explanted from a blastula embryo and cultured in isolation it develops into a ball of epidermis. But in the presence of a mesoderm inducing factor, the animal cap will differentiate into mesodermal derivatives, including notochord, muscle and blood. Members of the fibroblast growth factor family, in particular basic fibroblast growth factor (bFGF), and the transforming growth factor-β (TGF-β) family, notably activins and Vg-1, are potent inducers in this assay. Xenopus homologues of the Wnt gene family may also have a role in mesoderm induction. Both Xwntl (McMahon et al. (1989) Cell 58, 1075-1084) and Xwnt8 messenger RNAs elicit dorsal mesoderm formation when injected into the ventral side of an early embryo, an activity shared by Vg-1, and to a lesser extent by activin RNA. bFGF and activin protein′ can be detected in the early embryo and although there are no data on the localization of activin, there is evidence that bFGF is present in the marginal zone and vegetal pole of early blastula. Vg-1 is present at the appropriate time and in the right region known to be responsible for mesoderm induction in vivo. Although Xwntl and Xwnt8 are not present at the proper time or place to effect dorsal mesoderm induction, there may be other Xwnts that fulfill this role.
Many types of communication take place among animal cells. These vary from long-range effects, such as those of rather stable hormones circulating in the blood and acting on any cells in the body that possess the appropriate receptors, however distant they are, to the fleeting effects of very unstable neurotransmitters operating over distances of only a few microns. Of particular importance in development is the class of cell interactions called embryonic induction; this includes influences operating between adjacent cells or in some cases over greater than 10 cell diameters (Saxen et al. (1989) Int JDev Biol 33:21-48; and Gurdon et al. (1987) Development 99:285-306). Embryonic induction is defined as in interaction between one (inducing) and another (responding) tissue or cell, as a result of which the responding cells undergo a change in the direction of differentiation. This interaction is often considered one of the most important mechanism in vertebrate development leading to differences between cells and to the organization of cells into tissues and organs. Adult organs in vertebrates, and probably in invertebrates, are formed through an interaction between epithelial and mesenchymal cells, that is, between ectoderm/endoderm and mesoderm, respectively.
The effects of developmental cell interactions are varied. Typically, responding cells are diverted from one route of cell differentiation to another, by inducing cells that differ from both the uninduced and induced states of the responding cells (inductions). Sometimes cells induce their neighbors to differentiate like themselves (homoiogenetic induction); in other cases a cell inhibits its neighbors from differentiating like itself. Cell interactions in early development may be sequential, such that an initial induction between two cell types leads to a progressive amplification of diversity. Moreover, inductive interactions occur not only in embryos, but in adult cells as well, and can act to establish and maintain morphogenetic patterns as well as induce differentiation (J. B. Gurdon (1992) Cell 68:185-199).
While there has been considerable progress in identifying molecules responsible for mesoderm induction, practically nothing is known about the molecular nature of neural induction. Candidate neural patterners are growth factors that are involved in mesoderm patterning in earlier stages and become localized later in a subset of cells in the nervous system. These molecules include different members of the Wnt, TGF-β and FGF families. Three members of the Wnt family Wnt-1, Wnt-3 and Wnt-3A, are localized in the roof plate (dorsal spinal cord) and a subset of brain cells. Good evidence that Wnt products pattern the neural tube comes from homozygote mice lacking the Wnt-1 gene product; these mutant mice display a strong abnormality in the anterior hindbrain and posterior midbrain (a region that coincides with engrailed-2 expressing cells)(McMahon et al. (1992) Cell. 69:581-595). Vg-1, BMP-4 (Jones et al. (1991) Development. 111:532-542) and dorsalin-1 (Blumberg et al. (1991) Science 253:194-196) are examples or TGF-β family members that display restricted expression in the embryonic nervous system (see also, Lyons et al. (1991) Trends Genet 7:408-412; and Massague et al. (1990) J Biol Chem 265:21393-21396). Dorsalin-1 inhibits the differentiation of motor neurons and induces migration of neural crest cells and thus may be involved in dorsal ventral patterning of the neural tube (Blumberg et al. (1991) Science 253:194-196). Finally acidic FGF (aFGF), basic FGF (bFGF) as well as the newly characterized FGF from Xenopus embryos, XeFGF, (Isaacs et al. (1992) Development. 114:711-20) are all expressed in some cells of the developing neural tube (Weise et al. (1992) Cell & Tissue Research. 276:125-130; and Tannahill et al. (1992) Development. 115:695-702).
Since the natural embryonic neural inducer or patterner has yet to be characterized, the analysis of the mechanisms of induction and patterning is difficult. However, studies have demonstrated that notochord can induce and pattern neural structures (Jones et al. (1989) Development. 107:785-791; and Sharpe et al. (1987) Cell. 50:749-758) which implies that the signals can travel vertically from the axial mesoderm to the overlying ectoderm. The finding that neuralization can be induced by mesoderm suggests that neural induction involves a signal acting in a paracrine fashion, the transduction of which appears to involve protein kinase C (Otte et al. (1991) Science. 251:570-573). A recent series of experiments, exploring one of Spemann's original ideas, have demonstrated that signals involved in both induction and patterning of the nervous system can also travel through the plane of the ectoderm (Dixon et al. (1989) Development 106:749-757; Doniach et al. (1992) Science 257:542-545; and Ruiz i Altaba, A. (1992) Development. 115:67-80). It is now accepted that both types of mechanisms coexist in the embryo and play a role in neurogenesis.